docs: def _keplerian_to_cartesian(cls, coord, center):
          """Conversion from Keplerian to Cartesian coordinates
          """
  
          a, e, i, Ω, ω, ν = coord
  
          p = a * (1 - e ** 2)
          r = p / (1 + e * cos(ν))
          h = sqrt(center.µ * p)
          x = r * (cos(Ω) * cos(ω + ν) - sin(Ω) * sin(ω + ν) * cos(i))
          y = r * (sin(Ω) * cos(ω + ν) + cos(Ω) * sin(ω + ν) * cos(i))
          z = r * sin(i) * sin(ω + ν)
          vx = x * h * e / (r * p) * sin(ν) - h / r * (cos(Ω) * sin(ω + ν) + sin(Ω) * cos(ω + ν) * cos(i))
          vy = y * h * e / (r * p) * sin(ν) - h / r * (sin(Ω) * sin(ω + ν) - cos(Ω) * cos(ω + ν) * cos(i))
          vz = z * h * e / (r * p) * sin(ν) + h / r * sin(i) * cos(ω + ν)
  
          return np.array([x, y, z, vx, vy, vz], dtype=float)